<s>
A	O
frame	B-Protocol
check	I-Protocol
sequence	I-Protocol
(	O
FCS	O
)	O
is	O
an	O
error-detecting	B-Error_Name
code	I-Error_Name
added	O
to	O
a	O
frame	B-Protocol
in	O
a	O
communication	O
protocol	O
.	O
</s>
<s>
The	O
FCS	O
field	O
contains	O
a	O
number	O
that	O
is	O
calculated	O
by	O
the	O
source	O
node	O
based	O
on	O
the	O
data	O
in	O
the	O
frame	B-Protocol
.	O
</s>
<s>
This	O
number	O
is	O
added	O
to	O
the	O
end	O
of	O
a	O
frame	B-Protocol
that	O
is	O
sent	O
.	O
</s>
<s>
When	O
the	O
destination	O
node	O
receives	O
the	O
frame	B-Protocol
the	O
FCS	O
number	O
is	O
recalculated	O
and	O
compared	O
with	O
the	O
FCS	O
number	O
included	O
in	O
the	O
frame	B-Protocol
.	O
</s>
<s>
If	O
the	O
two	O
numbers	O
are	O
different	O
,	O
an	O
error	O
is	O
assumed	O
and	O
the	O
frame	B-Protocol
is	O
discarded	O
.	O
</s>
<s>
The	O
FCS	O
provides	O
error	B-Error_Name
detection	I-Error_Name
only	O
.	O
</s>
<s>
Error	B-Error_Name
recovery	I-Error_Name
must	O
be	O
performed	O
through	O
separate	O
means	O
.	O
</s>
<s>
Ethernet	O
,	O
for	O
example	O
,	O
specifies	O
that	O
a	O
damaged	O
frame	B-Protocol
should	O
be	O
discarded	O
and	O
does	O
not	O
specify	O
any	O
action	O
to	O
cause	O
the	O
frame	B-Protocol
to	O
be	O
retransmitted	O
.	O
</s>
<s>
Other	O
protocols	O
,	O
notably	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
,	O
can	O
notice	O
the	O
data	O
loss	O
and	O
initiate	O
retransmission	O
and	O
error	B-Error_Name
recovery	I-Error_Name
.	O
</s>
<s>
The	O
FCS	O
is	O
often	O
transmitted	O
in	O
such	O
a	O
way	O
that	O
the	O
receiver	O
can	O
compute	O
a	O
running	O
sum	O
over	O
the	O
entire	O
frame	B-Protocol
,	O
together	O
with	O
the	O
trailing	O
FCS	O
,	O
expecting	O
to	O
see	O
a	O
fixed	O
result	O
(	O
such	O
as	O
zero	O
)	O
when	O
it	O
is	O
correct	O
.	O
</s>
<s>
By	O
far	O
the	O
most	O
popular	O
FCS	O
algorithm	O
is	O
a	O
cyclic	O
redundancy	O
check	O
(	O
CRC	O
)	O
,	O
used	O
in	O
Ethernet	O
and	O
other	O
IEEE	O
802	O
protocols	O
with	O
32	O
bits	O
,	O
in	O
X.25	B-Protocol
with	O
16	O
or	O
32	O
bits	O
,	O
in	O
HDLC	O
with	O
16	O
or	O
32	O
bits	O
,	O
in	O
Frame	B-Protocol
Relay	I-Protocol
with	O
16	O
bits	O
,	O
in	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
(	O
PPP	O
)	O
with	O
16	O
or	O
32	O
bits	O
,	O
and	O
in	O
other	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
protocols	O
.	O
</s>
<s>
Protocols	O
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
tend	O
to	O
use	O
checksums	B-Algorithm
.	O
</s>
